Rename DatabaseBasedTest to AbstractJUnit3BaseTest

- Makes sense to do this now because we nowadays have a new class
AbstractJUnit4BaseTest, which mostly is a copypasta of this class here
but adapted to JUnit4. This class here is already deprecated by the
JUnit4 one, so it makes sense to give it a name which clearly makes it
visible what will be its replacement.
- Done using the Eclipse renaming tool.
- Reviewed for basic sanity.
- Manual grep done to ensure that every instance of the old name is
gone.